The seasonal and spatial variability of dissolved organic matter (DOM) quantity and chemical composition were investigated in the Yukon River basin of Alaska, United States, and northwestern Canada. ...Dissolved organic carbon (DOC), chromophoric DOM (CDOM), and dissolved lignin phenols were measured across a range of source waters and the seasonal hydrograph. Strong relationships were determined between CDOM and both DOC and lignin phenols, highlighting the potential for deriving detailed spatial and temporal distributions of DOM composition from CDOM monitoring. Maximum concentrations of measured parameters were observed during the spring flush, when DOM had a remarkably high content of aromatic vascular plant material derived from surface soil and litter layers. A larger portion of riverine DOM was attributed to vascular plant sources than previously believed by utilizing representative vegetation leachates and a soil pore water as end‐members. In combination with recent studies highlighting export of young, labile DOM during the spring flush in northern high‐latitude river systems, our results suggest riverine DOM is less degraded and more labile than previously thought with clear ramifications for its biomineralization or photo‐oxidation in marine environments.
Natural unsaturated-zone gas profiles at the U.S. Geological Survey's Amargosa Desert Research Site, near Beatty, Nevada, reveal the presence of two physically and isotopically distinct CO2 sources, ...one shallow and one deep. The shallow source derives from seasonally variable autotrophic and heterotrophic respiration in the root zone. Scanning electron micrograph results indicate that at least part of the deep CO2 source is associated with calcite precipitation at the 110-m-deep water table. We use a geochemical gas-diffusion model to explore processes of CO2 production and behavior in the unsaturated zone. The individual isotopic species 12CO2, 13CO2, and 14CO2 are treated as separate chemical components that diffuse and react independently. Steady state model solutions, constrained by the measured P(CO)2, delta13C (in CO2), and delta14C (in CO2) profiles, indicate that the shallow CO2 source from root and microbial respiration composes approximately 97% of the annual average total CO2 production at this arid site. Despite the small contribution from deep CO2 production amounting to approximately 0.1 mol m(-2) yr(-1), upward diffusion from depth strongly influences the distribution of CO2 and carbon isotopes in the deep unsaturated zone. In addition to diffusion from deep CO2 production, 14C exchange with a sorbed CO2 phase is indicated by the modeled delta14C profiles, confirming previous work. The new model of carbon-isotopic profiles provides a quantitative approach for evaluating fluxes of carbon under natural conditions in deep unsaturated zones.
Changing climate in northern regions is causing permafrost to thaw with major implications for the global mercury (Hg) cycle. We estimated Hg in permafrost regions based on in situ measurements of ...sediment total mercury (STHg), soil organic carbon (SOC), and the Hg to carbon ratio (RHgC) combined with maps of soil carbon. We measured a median STHg of 43 ± 30 ng Hg g soil−1 and a median RHgC of 1.6 ± 0.9 μg Hg g C−1, consistent with published results of STHg for tundra soils and 11,000 measurements from 4,926 temperate, nonpermafrost sites in North America and Eurasia. We estimate that the Northern Hemisphere permafrost regions contain 1,656 ± 962 Gg Hg, of which 793 ± 461 Gg Hg is frozen in permafrost. Permafrost soils store nearly twice as much Hg as all other soils, the ocean, and the atmosphere combined, and this Hg is vulnerable to release as permafrost thaws over the next century. Existing estimates greatly underestimate Hg in permafrost soils, indicating a need to reevaluate the role of the Arctic regions in the global Hg cycle.
Plain Language Summary
Researchers estimate the amount of natural mercury stored in perennially frozen soils (permafrost) in the Northern Hemisphere. Permafrost regions contain twice as much mercury as the rest of all soils, the atmosphere, and ocean combined.
Key Points
Permafrost stores a significant amount of mercury
Permafrost regions store twice as much mercury as all other soils, the ocean, and atmosphere combined
Thawing permafrost in a warming climate may release mercury to the environment
Landcover changes have altered the natural carbon cycle; however, most landcover studies focus on either forest conversion to agriculture or urban, rarely both. We present differences in dissolved ...organic carbon (DOC) concentrations and dissolved organic matter (DOM) molecular composition within Upper Mississippi River Basin low order streams and rivers draining one of three dominant landcovers (forest, agriculture, and urban). Streams draining forest and urban landcovers have greater DOC concentrations, likely driven by differences in carbon sourcing, microbial processing, and soil disturbance. Using Fourier transform-ion cyclotron resonance mass spectrometry, 24% of assigned molecular formulae are common across all landcovers. Relative abundances of N-,S- heteroatomic formulae (CHON, CHOS, CHONS) are higher for agricultural and urban streams, with agricultural stream DOM having more N-containing formulae compared to urban stream DOM, which has more S-containing formulae. Higher N-,S- heteroatomic formulae abundance, along with enrichment in aliphatic, N-aliphatic, and highly unsaturated and phenolic (low O/C) compound categories within agricultural and urban stream DOM are likely to result from increased anthropogenic inputs, autochthonous production, and microbial processing associated with agricultural and urban impacts. Reduced N-,S- heteroatomic formulae abundances in forested stream DOM, along with enrichments in condensed aromatics, polyphenolics, and highly unsaturated phenolic (high O/C) compound categories, likely reflect greater contributions from surrounding organic-rich forest soil and vegetation. Overall, landcover change from forested to agriculture lowers DOC concentrations and changes from forested to agriculture or urban increases autochthonous, and presumably more biolabile, DOM contributions with ramifications for stream biogeochemical cycling.
The quality and quantity of dissolved organic matter (DOM) exported by Arctic rivers is known to vary with hydrology and this exported material plays a fundamental role in the biogeochemical cycling ...of carbon at high latitudes. We highlight the potential of optical measurements to examine DOM quality across the hydrograph in Arctic rivers. Furthermore, we establish chromophoric DOM (CDOM) relationships to dissolved organic carbon (DOC) and lignin phenols in the Yukon River and model DOC and lignin loads from CDOM measurements, the former in excellent agreement with long‐term DOC monitoring data. Intensive sampling across the historically under‐sampled spring flush period highlights the importance of this time for total export of DOC and particularly lignin. Calculated riverine DOC loads to the Arctic Ocean show an increase from previous estimates, especially when new higher discharge data are incorporated. Increased DOC loads indicate decreased residence times for terrigenous DOM in the Arctic Ocean with important implications for the reactivity and export of this material to the Atlantic Ocean.
Determining the discharge of ground water to Shingobee Lake (66 ha), north‐central Minnesota, is complicated by the presence of numerous springs situated adjacent to the lake and in the shallow ...portion of the lakebed. Springs first had to be located before these areas of more rapid discharge could be quantified. Two methods that rely on the distribution of aquatic plants are useful for locating springs. One method identifies areas of the near‐shore lakebed where floating‐leaf and emergent aquatic vegetation are absent. The second method uses the distribution of marsh marigold (Caltha palustris L.) to locate springs that discharge on land near the shoreline of the lake. Marsh marigold produces large (2 to 4 cm diameter) yellow flowers that provide a ready marker for locating ground water springs. Twice as many springs (38) were identified using this method as were identified using the lack of near‐shore vegetation. A portable weir was used to measure discharge from onshore springs, and seepage meters were used to measure discharge from near‐shore springs. Of the total 56.7 L s−1 that enters the lake from ground water, approximately 30% comes from onshore and near‐shore springs.
•Examined landcover impacts on stream dissolved organic matter (DOM) bioavailability.•Streams draining urban lands contained more bioavailable DOM than agricultural lands.•Forested streams had high ...organic carbon concentrations but lowest bioavailability.•Demonstrated relationship between aliphatic compound abundances and bioavailability.•Landcover does not impact what compounds are lost during bioincubation.
Anthropogenic conversion of forests and wetlands to agricultural and urban landcovers impacts dissolved organic matter (DOM) within streams draining these catchments. Research on how landcover conversion impacts DOM molecular level composition and bioavailability, however, is lacking. In the Upper Mississippi River Basin (UMRB), water from low-order streams and rivers draining one of three dominant landcovers (forest, agriculture, urban) was incubated for 28 days to determine bioavailable DOC (BDOC) concentrations and changes in DOM composition. The BDOC concentration averaged 0.49 ± 0.30 mg L−1 across all samples and was significantly higher in streams draining urban catchments (0.72 ± 0.34 mg L−1) compared to streams draining agricultural (0.28 ± 0.15 mg L−1) and forested (0.47 ± 0.17 mg L−1) catchments. Percent BDOC was significantly greater in urban (10% ± 4.4%) streams compared to forested streams (5.6% ± 3.2%), corresponding with greater relative abundances of aliphatic and N-containing aliphatic compounds in urban streams. Aliphatic compound relative abundance decreased across all landcovers during the bioincubation (average -4.1% ± 10%), whereas polyphenolics and condensed aromatics increased in relative abundance across all landcovers (average of +1.4% ± 5.9% and +1.8% ± 10%, respectively). Overall, the conversion of forested to urban landcover had a larger impact on stream DOM bioavailability in the UMRB compared to conversion to agricultural landcover. Future research examining the impacts of anthropogenic landcover conversion on stream DOM composition and bioavailability needs to be expanded to a range of spatial scales and to different ecotones, especially with continued landcover alterations.
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Riverine ecosystems receive organic matter (OM) from terrestrial sources, internally produce new OM, and biogeochemically cycle and modify organic and inorganic carbon. Major gaps remain in the ...understanding of the relationships between carbon sources and processing in river systems. Here we synthesize isotopic, elemental, and molecular properties of dissolved organic carbon (DOC), particulate organic carbon (POC), and dissolved inorganic carbon (DIC) in the Upper Mississippi River (UMR) system above Wabasha, MN, including the main stem Mississippi River and its four major tributaries (Minnesota, upper Mississippi, St. Croix, and Chippewa Rivers). Our goal was to elucidate how biological processing modifies the chemical and isotopic composition of aquatic carbon pools during transport downstream in a large river system with natural and man‐made impoundments. Relationships between land cover and DOC carbon‐isotope composition, absorbance, and hydrophobic acid content indicate that DOC retains terrestrial carbon source information, while the terrestrial POC signal is largely replaced by autochthonous organic matter, and DIC integrates the influence of in‐stream photosynthesis and respiration of organic matter. The UMR is slightly heterotrophic throughout the year, but pools formed by low‐head navigation dams and natural impoundments promote a shift toward autotrophic conditions, altering aquatic ecosystem dynamics and POC and DIC compositions. Such changes likely occur in all major river systems affected by low‐head dams and need to be incorporated into our understanding of inland water carbon dynamics and processes controlling CO2 emissions from rivers, as new navigation and flood control systems are planned for future river and water resources management.
Key Points
Dissolved organic carbon composition is linked to basin forest area and wastewater inputs and shows little in‐stream biological modification
Biological processes modify dissolved inorganic carbon isotopic composition, so that it deviates from terrestrial carbon source composition
Low‐head dams and reservoirs shift fluvial systems toward autotrophic conditions, reflected in particulate organic carbon composition
Plain Language Summary
Watersheds of the Upper Mississippi River Basin drain a region of diverse land use types, ranging from heavily agricultural to forested. This study investigates links between the chemical features of carbon carried by these rivers and different land use types within their basins. These features are used to identify signals of biological activity during downstream transport, which influence how much carbon dioxide the river releases to the atmosphere. We found that certain portions of the carbon carried by the Upper Mississippi River maintain markers of land use, while other portions reflect the activity of photosynthesis and respiration. River impoundments such as the natural Lake Pepin on the Mississippi River main stem remove light‐blocking particles, thus shifting the river's balance from respiration toward photosynthesis. These results are important for understanding how rivers may respond to future changes in land use, climate, and other environmental conditions, especially given the expected global increase in river navigation structures.
High‐elevation lakes in the western United States are sensitive to atmospheric deposition of sulfur and nitrogen due to fast hydrologic flushing rates, short growing seasons, an abundance of exposed ...bedrock, and a lack of well‐developed soils. This sensitivity is reflected in the dilute chemistry of the lakes, which was documented in the U.S. Environmental Protection Agency's Western Lake Survey of 1985. Sixty‐nine lakes in seven national parks sampled during the 1985 survey were resampled during fall 1999 to investigate possible decadal‐scale changes in lake chemistry. In most lakes, SO4 concentrations were slightly lower in 1999 than in 1985, consistent with a regional decrease in precipitation SO4 concentrations and in SO2 emissions in the western United States. Nitrate concentrations also tended to be slightly lower in 1999 than in 1985, in contrast with generally stable or increasing inorganic N deposition in the west. Differences in alkalinity were variable among parks but were relatively consistent within each park. Possible effects of annual and seasonal‐scale variations in precipitation amount on lake chemistry were evaluated based on climate data available for the parks and an analysis of climatic effects at two research watersheds with long‐term records. Results suggest that rain prior to sampling in 1985 may have caused elevated NO3 in some lakes due to direct runoff of precipitation and flushing of NO3 from alpine soils, which may explain some of the decrease in NO3 concentrations observed in survey lakes.
Carbon dioxide (CO
2
) emissions from rivers and other inland waters are thought to be a major component of regional and global carbon cycling. In large managed rivers such as the Columbia River, ...contemporary ecosystem changes such as damming, nutrient enrichment, and increased water residence times may lead to reduced CO
2
concentrations (and emissions) due to increased primary production, as has been shown in another large North American river (Upper Mississippi). In this work, spatial patterns of water quality, including dissolved CO
2
concentrations, were assessed in the Lower Columbia River (LCR) and major tributaries using underway measurements from a small research vessel during July 2016. We observed near-equilibrium CO
2
conditions and overall weak supersaturation of CO
2
in the main channel (average 133.8% saturation) and tributaries. We observed only weak correlations between CO
2
saturation, chlorophyll a fluorescence, and turbidity, thus not strongly supporting our hypothesis of primary productivity controls. In general, the LCR was clear (low turbidity, mean = 1.48 FNU) and had low chlorophyll fluorescence (mean = 0.177 RFU) during the sampling period. As a whole, the LCR was homogeneous with respect to biogeochemical conditions and showed low spatial variability at >100 km scales. Overall, we find that the LCR is likely a weak summertime source of CO
2
to the atmosphere, in line with findings from other altered rivers such as the Upper Mississippi.
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